You might not be able to tell with a casual look, but rivers across the U.S. have been changing color. And that’s not necessarily a good thing. A recent study shows that of the tens of thousands of mile-long river segments across the country, approximately a third have noticeably shifted color in satellite images during the last 35 years. More than 11,000 miles became greener or moved toward the violet end of the spectrum, and people are the cause of many of the shifts.